Devices able to extend to force two objects apart come in a number of forms. One common example is a device used to extend the distance between an object and the ground on which it rests, thereby raising the object, i.e. a jack. Jacks are used to lift objects off the ground. They are used in many situations, although raising vehicles is perhaps the most well-known use. Raising a vehicle off the ground enables maintenance or repairs to be carried out, for example changing a tyre or by allowing easy access to the underside of the vehicle. Jacks may also be used to, for example, lift houses off their foundations.
Some kinds of jacks use mechanical advantage to allow a vehicle (or other object) to be lifted by manual force alone. For example, screw jacks operate by the manual winding of a screw in order to raise the jack. Some such jacks may be conveniently stored in a vehicle for emergencies.
Hydraulic jacks operate on the principle of injecting an incompressible fluid into a chamber below a piston. This causes the piston to rise up out of a housing. The top of the piston pushes an object upwards as it rises.
As well as lifting a load, devices are also required to safely maintain the load in the lifted position for an unspecified duration of time, after which the load may be released and safely lowered. In some instances, one piece of equipment is used to lift the load and a separate apparatus is used to support the load in the lifted position. In Australia and New Zealand, different safety standards govern the two functions and separate pieces of equipment for each function allows the respective equipment to be tailored to conform to the relevant standards.
Some jacks are available that can perform both lifting and supporting functions to the respective standards. This is advantageous as only a single device is required, saving on space and handling. Also, some loads have only a limited number of locations suitable for a lifting or supporting force to be applied so finding two locations for fitting separate lift and support devices can be difficult.
Integrated lift and support devices often take the form of conventional jacks incorporating fail-safe devices to support the load in the event the mechanism supporting the jacking function malfunctions. Such fail-safes may comprise mechanical locking mechanisms so that, even if the hydraulic mechanism fails, the piston is maintained at a certain height unless the mechanical lock is disengaged.
Some prior art jacks have locking mechanisms located externally to the device. In such devices, the locking mechanisms are vulnerable to contamination, corrosion or mechanical damage. External mechanisms can also expose operators and other equipment to dangerous entrapment or pinching.
U.S. Pat. No. 2,540,578 discloses a hydraulic jack in which a piston is extendable out of a cylinder upon injection of hydraulic fluid into the chamber below the piston. The cylinder can be locked in an extended position by engagement of a locking device mounted on the inside of the cylinder with ratchet grooves on a post inside the piston. The post only has ratchet grooves down opposing sides. Therefore to release the locking mechanism, the locking device is rotated through 90° by means of a handle, thereby moving the locking device out of alignment with the grooves and allowing the piston to freely move up and down the cylinder and post. Although some parts of the locking mechanism of this jack are internal to the piston, the handle is external and could be vulnerable to being knocked, releasing the lock. In addition, an operator must manually turn the handle to lock/unlock the mechanism. Manual locking mechanisms in general are vulnerable to user error. To reach the handle may require the operator to put part of their body under the supported load, which could be unsafe.
U.S. Pat. No. 5,205,203 discloses a hydraulic cylinder unit intended to be used in a pantograph-type vehicle jack. The cylinder comprises a piston which extends out of a cylinder when hydraulic fluid is injected through an aperture into the chamber below the piston. Inside the piston is a rod fixed to the cylinder. A locking mechanism between the piston and rod is provided in a flange on the piston. The locking mechanism is operated by hydraulic fluid in the cylinder and is therefore prone to failure in the event of fluid loss in the cylinder.
It is an object of the invention to provide an improved jack that can be used to safely lift and support a load. Alternatively, it is an object to address at least some of the aforementioned problems of the prior art. Alternatively, it is an object of the invention to at least provide the public with a useful choice.